36th
International Electronic Manufacturing Technology Conference, 2014
Ultra Low Loop Wire Bonding of 20 µµµµm Palladium Coated Copper Wire for Very Thin Packages
Loh Kian Hwa, Loh Lee Jeng, Liew Siew Har, Eric Erfe
Carsem Technology Center (CTC)
Carsem (M) Sdn Bhd (S-site)
Lot 52986, Taman Meru Industrial Estate, Jelapang
P.O. Box 380, 30720 Ipoh, Perak, Malaysia
Abstract
In 1998, Carsem introduced the MLP (Micro Leadless
Package). Today, the MLP has become the semiconductor
industry’s package of choice for many devices with low-to-
medium I/O count. Coincidentally, the MLP came out at the
same time that smartphones were just starting to take off.
Following the tremendous increase in market demand for
smartphones, tablets and other handheld devices, the MLP
soon outpaced the growth (in billions of units shipped) of
every other leadframe based package in the industry. This
phenomenal growth is greatly attributed to the MLP’s low
cost, small size, and excellent thermal & electrical
performance characteristics.
Consumer handheld devices are constantly getting
thinner, smaller and cheaper. In recent years, there has been
a significant trend towards thinner and thinner packages.
Nowadays, the MLP package is available in different
package thicknesses ranging from 0.9 mm to as thin as 0.3
mm. The trend for thinner MLP presents several challenges
in the choice of packaging materials, most especially in the
choice of interconnect wire material. Gold wire has been
widely used by the industry to interconnect the silicon chip
to the metal leadframe. Thin MLPs will require wire bonds
with lower loop profiles. Numerous innovations have been
implemented in the gold wire bonding process. For instance,
forward bond loops with 50 µm loop height are already in
mass production. Other innovations such as die to die
bonding, cascade bonding, and bonding overhang die are
also being implemented.
But as the price of gold continues to sky rocket, copper
wire will gain widespread use, especially for price-sensitive
consumer electronic devices. Therefore, it is only inevitable
that the technological developments made on low loop wire
bonding of gold wires will have to be extended to copper
wires as well.
This paper presents the development work done on Ultra
Low Loop wire bonding using 20 µm (0.8 mil) palladium
coated copper wire. Actual results will be shared to
demonstrate capability to achieve a maximum loop height of
63.5 µm (2.5 mils). This paper will also discuss details on
the loop type selection, the appropriate test vehicles used,
and the relevant output responses after wirebond. And lastly,
results from the reliability stress tests will also be discussed.
1. Introduction
In order to produce wires with very low loops (for
example, 2.5 mils) using very fine (ex: 0.8 mil) palladium-
coated copper wire, there are a several looping profiles
available such as the flex loop, the escargot loop, and the
folded loop (just to name a few). For this study, the team
selected to use the folded loop method. The selection was
based on Carsem’s long experience using the folded loop
method to produce ultra-low loops using fine gold wire. And
between the 3 methods mentioned, the folded loop induces
the least amount of stress at the neck of the wire.
Fig 1: Flex loop
Fig 2 : Escargot loop
Fig 3 : Folded loop
2. First test – Dummy bonding on leadframe
To demonstrate proof of concept, we performed dummy
bonding on bare leadframe without any die attached. Wires
of varying lengths were bonded using the folded loop
method. The length of the wires ranged from 1mm to 4mm
since these are the typical wire lengths we would expect in a
9x9 QFN package. And the target output responses are as
follows:
1) Loop height below 2.5mils (63.5µm)
2) No neck crack for the first kink
3) Looping consistency
4) Min bond pull strength of 2.5gf
5) No wire sway
Fig 4: First test device bonding layout
3. First test result
We were able to achieve less than 2.5mils (63.5um) loop
height. This is based on a sample size of
wire length.
Fig 5: First test result table
Fig 6: JMP comparison of loop height results (1
on leadframe
performed dummy
bonding on bare leadframe without any die attached. Wires
using the folded loop
wires ranged from 1mm to 4mm
since these are the typical wire lengths we would expect in a
And the target output responses are as
Loop height below 2.5mils (63.5µm)
able to achieve less than 2.5mils (63.5um) loop
of 30 readings per
JMP comparison of loop height results (1
st test)
4. First test result discussion
The Heat Affected Zone (HAZ) has a direct
the minimum loop height you can produce. The HAZ of
gold wires are well documented, but there is n
defined HAZ for palladium coated copper wire. Despite this,
the results from our first test have demonstrated that
(20um) palladium coated copper wire can be bent as low as
gold wire in order to meet the loop height
50um at the neck.
Fig 7: Gold versus Palladium Coated Copper properties
comparison table
5. Second test study – Bonding on daisy chain die
For the second test, we bonded on a 1x1mm daisy chain
die on a 5x5mm MLP package with 3x3mm
paddle. The die was purposely attached at
order to assess different wire lengths
~4mm). The die thickness is 50um die
bondline thickness is 25um. This was chosen in order to
assess the most difficult bond condition
target responses are the same as the first test.
Fig 7: Second test device bonding layout.
6. Second test loop height result
We are able to achieve less than 2.5mils (63.5um) loop
height with folded loop for wire length fr
~4mm. This is based on a sample size of 613 wires.
The Heat Affected Zone (HAZ) has a direct impact on
the minimum loop height you can produce. The HAZ of
gold wires are well documented, but there is no clearly
defined HAZ for palladium coated copper wire. Despite this,
have demonstrated that 0.8mils
alladium coated copper wire can be bent as low as
gold wire in order to meet the loop height target of less than
Fig 7: Gold versus Palladium Coated Copper properties
Bonding on daisy chain die
, we bonded on a 1x1mm daisy chain
with 3x3mm die attach
attached at top left corner in
s (ranging from ~1mm to
50um die and the epoxy
. This was chosen in order to
condition. And the output
target responses are the same as the first test.
Fig 7: Second test device bonding layout.
We are able to achieve less than 2.5mils (63.5um) loop
for wire length from ~1mm to
This is based on a sample size of 613 wires.
36th
International Electronic Manufacturing Technology Conference, 2014
Fig 8: Loop height result table (2
nd test)
Fig 9: JMP comparison chart of loop height results (2
nd test)
7. Second test wire pull strength result
Aside from the samples that were prepared using folded
loop, we also assembled a few units with the traditional
square loop. And we compared the wire pull strength for
additional information.
Fig 10: Wire pull strength result table
Fig 11: JMP comparison chart of wire pull strength (2
nd test)
The wire pull strengths of both folded loop and square
loop can meet the minimum spec limit with Cpk > 1.67.
The average pull strength of folded loop is about 27% lower
compare to standard square loop (~4mils loop height). And
the failure mode for the folded loop 44% break at the neck
while the square had no failures at the neck. This difference
is possibly due to the reduced cross sectional area of the
neck of a folded loop.
Fig 12: Wire pull failure mode histogram
Fig 13: SEM image of a folded loop after wire pull test
36th
International Electronic Manufacturing Technology Conference, 2014
SEM inspection revealed no visible crack at wire neck
area, looping is consistent and no wire sway. Optical
inspection revealed that the Palladium coating is scratched
off by the wirebond capillary. There is partial exposure of
the base copper at the folded region (neck area). To assess
the reliability risk, samples were submitted for Reliability
testing.
Fig 14: SEM image on the ultra low loop
Fig 15: Optical inspection
Fig 16: Folded loop versus Square loop
8. Reliability test result
We have selected 2 test vehicles (32-lead MLPQ 5x5mm
with package thickness of 0.4mm and 16-lead MLPQ
2.3x2.3mm with package thickness of 0.32mm)
manufactured from 2 sites (Carsem Ipoh and Carsem
Suzhou) for reliability test. All samples passed MSL1,
260°C, uHAST 96hours, -65 +150°C condition C
Temperature Cycle 1000 cycles and 150°C High
Temperature Storage 1008hours.
Fig 17: Reliability Test summary table
To confirm the robustness of this 2.5mils (63.5um) ultra
low loop, we also subjected unmolded strips to Temperature
Cycling up to 1000 cycles under -65°C +150°C to see if
there will be any significant degradation of wire mechanical
properties. The results obtained also show wire pull Cpk >
1.67 with acceptable break mode.
Fig18: Unmolded strip wire pull test result after 1000
temperature cycles
Fig 19: JMP comparison chart of unmolded strip wire
pull test after 1000 Temp Cycles
36th
International Electronic Manufacturing Technology Conference, 2014
Fig 20: Wire pull test failure mode distribution after
1000 Temperature Cycles
9. Discussion
Based on our high volume manufacturing experience
with gold wire, we have characterized the Palladium Coated
copper wire thoroughly using appropriate test vehicles. The
results obtained from the first study on bare leadframe
bonding, the second study using 50um thickness test die
bonding and lastly the reliability test and aging test helped
us to develop the suitable bonding parameters for a robust
process for Palladium Coated copper wire.
10. Conclusion
Ultra low loop (folded loop) is feasible for Palladium
Coated Copper wire based on these assessment results:
a) Loop height below 2.5mils (63.5µm) with Ppk >
1.67
b) No neck crack for the 1st kink
c) Looping is consistent
d) Passed minimum bond pull strength of 2.5gf with
Ppk > 1.67
e) No wire sway
The average wire pull strength for folded loop is about
27% lower as compared to standard square loop (~4mils
loop height). This is possibly due to the reduced cross
sectional area of the neck of a folded loop.
Reliability tests and aging tests have confirmed there is
no significant degradation of the wire’s mechanical
properties after environmental stress test.
In closing, this new process capability of ultra low loop
wire bonding will enable the introduction of next-gen ultra-
thin QFN such as the X3.2 package: Carsem’s latest MLP
package introduced in 2014.
Fig 21: 0.9mm package with normal loop versus 0.32mm
package with ultra low loop
Acknowledgments
The authors would like to thank to Carsem Technology
Officer, Mr. LW Yong and Senior R & D Manager, Mr. KH
Lee to initiate this project..
References
1. SH Liew, WL Law “Reliable Ultra-low-loop Bonding
Approach on X2/X3 thin QFN”, 35th International
Electronic Manufacturing Technology Conference,
2012.
2. G. Harman, “Wire bonding in Microelectronics
material, process, reliability and yield”, 2nd
adition,
McGraw-Hill, New York (1997), pp. 203-207.
Normal loop height
4 to 5 mils
(100 to 125 um)
V package
thickness
0.9 mmUltra-low
loop height<2.5 mils
(<63.5 um)
X3.2
package thickness
0.32 mm
Ultra Thin QFN
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